1. Field of the Invention
The present invention generally relates to the field of audio and video processing. More specifically, the present invention relates to a method of generating timestamps for isochronous data.
2. Background Information
One characteristic that all isochronous transport mechanisms share is the ability to transmit data within certain fixed time constraints whereby maximum data latency and minimum bandwidth allocation is guaranteed. Isochronous transport mechanisms are useful, for example, in the transfer of multimedia data where synchronization of audio and video upon playback is important. To that end, a sound or picture transmitted from a device such as a digital video camera, across a network to a display device, for example, should be received at the display device at nearly the same rate the data was transmitted from the video camera.
Isochronous networks typically grant the highest priority access to a device known as a cycle master that maintains a common clock source for the network. Timing information is exchanged within isochronous networks through the use of timestamps within network packets. Using timestamps, the cycle master transmits to all isochronous nodes, a periodic timing request known as a “cycle start” indicating the start of each isochronous cycle. Each node with isochronous service contains a 32-bit “cycle timer register”. The low-order 12-bits of the register represent a modulo 3072 count, which increments once every 24.576 MHz clock period. The next 13 higher order bits represent a count of 8 kHz cycles, and the highest 7 bits represent a count of seconds. If a cycle start is delayed, the amount of time that the cycle start was delayed is encoded within the data packet which is broadcast to the cycle timer register of each node on the network. The cycle master copies its cycle timer register to all isochronous nodes along with the cycle start request, thereby synchronizing all nodes to be within a constant phase difference.
The IEEE Standard for a High Performance Serial Bus, IEEE Std. 1394-1995 published Aug. 30, 1996 (1394-1995 Standard) and its progeny provide a high speed serial protocol which permits implementation of high speed data transfers of both asynchronous and isochronous data. The existing progeny includes P1394a Draft Standard for a High Performance Serial Bus (1394a Standard) and P1394b Draft Standard for a High Performance Serial Bus (1394b Standard). Generically, networks implementing 1394-1995, 1394a, 1394b or subsequent revisions and modifications thereof are referred to herein as IEEE 1394 networks. Additionally, information regarding a general isochronous packet format used in conjunction with IEEE 1394 may be found in the International Electrotechnical Commission standard 61883-1:1998-02 (hereinafter “IEC 61883”).
In order to preserve data ordering on an IEEE 1394 network, each isochronous packet of audio/video data is time-stamped before it is transmitted. Each timestamp is based upon the current cycle-time of the isochronous network as determined by the cycle master. It is possible, however, that the audio/video data may have been generated based upon an external reference signal (e.g. a house reference signal) having an operating frequency that is asynchronous to that of the cycle master. Because isochronous packets are consumed (i.e. replayed) at destination nodes according to the packet's respective isochronous timestamps independent of the frequency at which the data was originally generated, audio/video playback may be adversely affected if the reference clocks and IEEE 1394 clocks are allowed to drift from one another. Furthermore, because isochronous packets are based on a first time domain as determined by the cycle master, and audio/video data is based upon a second time domain, typically a house reference signal, it is often difficult to accurately reconstruct audio/video data after it has been transmitted over an isochronous network.